Lead-acid batteries are used as starters for automobiles. Lead-acid batteries are also widely used as backup power supplies for industrial and commercial devices. Currently, there is a growing move to replace lead-acid batteries used as backup power supplies with nickel-metal hydride batteries or lithium ion batteries, which mainly originates from demands for smaller power supplies by replacing lead-acid batteries with batteries having a high energy density and demands for environmentally friendly batteries by removing batteries using lead in order to reduce environmental burdens.
As for lead-acid batteries used for automobiles, there is no such move. In order to reduce environmental burdens, however, it is desirable to replace lead-acid batteries with environmentally friendly batteries. Considering the application in automobiles, it is also desirable to replace nickel-metal hydride batteries with lighter and superior lithium ion batteries. In view of the above, the replacement of lead-acid batteries currently used in the fields of automobiles and backup power supplies with lithium ion batteries will certainly be necessary in the future.
Lithium ion batteries utilized as the main power supplies for mobile communication devices and portable electronic devices are characterized by having a high electromotive force and a high energy density. The positive electrode active materials for lithium ion batteries include lithium cobaltate (LiCOO2), lithium nickelate (LiNiO2), manganese spinel (LiMn2O4) and the mixtures thereof. These positive electrode active materials have a voltage of not less than 4 V relative to that of lithium.
As the negative electrodes therefor, carbon materials are typically used. The combination of the above-described positive electrode and the negative electrode made of carbon forms a 4 V level lithium ion battery. Desirable alternatives to lead-acid batteries are lithium ion batteries having a voltage equal to that of lead-acid batteries and lithium ion batteries having a voltage in multiples of 6 when a plurality of them are connected in series. From this point of view, 4 V level lithium ion batteries are not suitable for that purpose. As an alternative to lead-acid batteries, lithium ion batteries having a voltage of 2 or 3 V are suitable. Although there are various methods for reducing voltage, the prior art examples particularly dealing with the system that employs a titanium oxide in the negative electrode will be described below because the present invention is characterized by using a titanium oxide in the negative electrode.
Japanese Laid-Open Patent Publication No. Hei 07-320784 discloses a non-aqueous electrolyte lithium ion battery including a negative electrode containing a lithium-titanium oxide with a spinel-type structure (Li4/3Ti5/3O4) as a negative electrode active material and a positive electrode containing Li2MnO3 or LiMnO2 as a positive electrode active material, and a non-aqueous electrolyte. The disclosed battery, however, has an actual discharge voltage of around 1.6 V, which is quite low for a 2 V battery.
Japanese Laid-Open Patent Publication No. Hei 10-027609 discloses a non-aqueous electrolyte secondary battery employing a lithium-titanium oxide with a spinel-type structure as a negative electrode active material and an active material containing a lithium-manganese oxide (Li4/3Mn5/3O4) with a spinel structure in a positive electrode. This battery system has a voltage of around 2.5 V, which could be a preferred voltage, but it is slightly high for a 2 V level battery. This prior art example, however, does not fundamentally solve the problem of deterioration during storage and a low cycle life caused by dissolving of Mn from the spinel-type lithium-manganese oxide.
Japanese Laid-Open Patent Publication No. Hei 07-335261 discloses a battery using a lithium cobaltate (LiCoO2) as a positive electrode active material and a lithium titanate (Li4/3Ti5/3O4) as a negative electrode active material. This battery system has a preferred voltage, but increased cost is inevitable since a large amount of Co metal is used. Additionally, problems are expected to occur in terms of long-term reliability because LiCoO2 repeatedly expands and contracts in volume during charge and discharge, which facilitates the destruction of crystal lattice as will be described later.
Japanese Laid-Open Patent Publication No. Hei 10-027626 discloses a lithium secondary battery using, as a positive electrode active material, a lithium transition metal oxide represented by LiAxB1−xO2, where A and B are metal elements selected from Co, Ni, Mn, Fe, V, Al and Ti, and a lithium-titanium oxide represented by Li4/3Ti5/3O4 as a negative electrode active material, wherein the actual capacity ratio of the lithium titanium oxide to the lithium transition metal oxide is not greater than 0.5. The invention disclosed in this patent publication pertains to a battery design focusing primarily on the capacity balance of positive and negative electrodes, and the necessity of the combinations that make full use of the characteristics of each material as will be described later is not mentioned.
Japanese Laid-Open Patent Publication No. Hei 10-069922 discloses a battery system using a titanium ion-deficient type lithium titanium oxide Li4/3Ti5/3O4 having a spinel structure, in a negative electrode, wherein at least one of carbon, graphite, WO2, Fe2O3, LiFe5O8, SiO2 and SnO is added. This invention is intended to improve the resistance against overcharge and overdischarge, which differs from the present invention in terms of constituent and purpose.
Japanese Laid-Open Patent Publication No. 2001-210324 discloses, as a positive electrode active material, a lithium manganese composite oxide represented by Li1+xMyMn2−x−yO4−z, where M is one or more of Ti, V, Cr, Fe, Co, Ni, Zn, Cu, W, Mg and Al, 0≦x≦0.2, 0≦y<0.5 and 0≦z<0.2, wherein the half width of the (400) diffraction peak obtained by powder X-ray diffraction using CuKα radiation is not less than 0.02 θ and not greater than 0.1 θ (θ is an angle of diffraction), and the primary particles thereof has an octahedron shape. This patent publication proposes a battery comprising a positive electrode containing the above-described positive electrode active material and a negative electrode containing, as a negative electrode active material, a lithium titanium composite oxide represented by LiaTibO4, where 0.5≦a≦3 and 1≦b≦2.5. This battery system has a discharge voltage of not less than 3 V, which differs from the voltage of the battery of the present invention.
Japanese Laid-Open Patent Publication No. 2001-243952 discloses, as a positive electrode active material, a lithium nickel composite oxide represented by Li1−xAxNi1−yMyO2, where A is one or more selected from alkaline metals except Li and alkaline-earth metals; M is one or more selected from Co, Mn, Al, Cr, Fe, V, Ti and Ga; 0≦x≦0.2; and 0.05≦y≦0.5, wherein the primary particles with an average particle size of not less than 0.5 μm aggregate to form secondary particles. This patent publication also discloses a battery including a positive electrode containing the above-described positive electrode active material and a negative electrode containing, as a negative electrode active material, a lithium titanium composite oxide represented by LiaTibO4, where 0.5≦a≦3 and 1≦b≦2.5. The object of this disclosed invention is to provide an inexpensive lithium secondary battery having good cycle characteristics particularly during high temperature storage and good high temperature storage characteristics.
For that purpose, the invention of this patent publication focuses on the following two points. Firstly, it is disclosed that the primary particles of the positive electrode active material are required to have an average particle size of not less than 0.5 μm in the invention of the patent publication. The conception thereof is based on inherently unavoidable volume change. There is described in the specification that the primary particles are nearly identical to monocrystals and they expand and contract due to the absorption and desorption of lithium during the repetition of charge and discharge, which is interpreted that the volume change cannot be avoided. In order to minimize the volume change, it is proposed to increase the particle size of the primary particles.
As for the selection of the transition metal, although the patent publication teaches to add Co, Mn, Al or the like to prevent the phase transition of the crystal structure from a hexagonal system into an monoclinic phase, it was known at the time the invention was made and was not a special technique. The patent publication further teaches that the addition of Al prevents the decomposition reaction of the active material accompanied by the release of oxygen and improves thermal stability and electron conductivity, but this was also known at the time the invention was made. For example, J. Electrochem. Soc., 140, 1862 (1993) by T. Ohzuku et al. and Japanese Laid-Open Patent Publication No. Hei 09-171824 teach to prevent the phase transition of a crystal by partly replacing Ni of LiNiO2 with Co. J. Electrochem. Soc., 142, 4033 (1995) by T. Ohzuku et al. and Japanese Laid-Open Patent Publication No. Hei 10-208744 teach to improve thermal stability by adding Al.
Secondly, Japanese Laid-Open Patent Publication No. 2001-243952 proposes the use of a lithium titanium oxide represented by LiaTibO4 (0.5≦a≦3 and 1≦b≦2.5) as a negative electrode active material. It is disclosed that this prevents the decomposition of a non-aqueous electrolyte and the deposition of a reaction product onto the surface of a negative electrode accompanied by the decomposition, which improves the cycle life, because the reduction potential of the lithium titanium oxide is 1.5 V relative to that of Li/Li+, higher than that of a typically used carbonaceous material. The fact that the lithium titanium oxide has a high potential has already been explicitly disclosed in the aforementioned prior art examples of titanium oxides, and it is not something new that the above-mentioned invention offers.
Finally, Japanese Laid-Open Patent Publication No. 2001-243952 is totally silent on the point that the combination according to the patent publication produces a new effect. It only discloses the individual effect of the positive and negative electrodes and merely lists the combinations of effects, which are easily conceivable from prior arts. Moreover, it discloses a core material for negative electrode made of copper as shown in EXAMPLE, a separator made of thin microporous film, and an electrolyte including currently widely used solvent and solute. All of them are known materials and used in currently commercially available lithium secondary batteries. Additionally, it does not at all teach that a new effect is produced by specifically selecting them.
In recent years, demand is getting stronger, particularly for improved high rate and pulse characteristics. Charging/discharging at a high rate results in a higher load to a material so that the prior arts have failed to improve the factors for structural damage, etc. It is also getting difficult to maintain the current level of cycle life. Moreover, the use of lithium cobaltates or graphite materials having a layered structure is accompanied by problems, particularly short cycle life at a high rate charge/discharge caused by leakage of an electrolyte from between electrodes or by material stress caused by the repetition of expansion and contraction of lithium cobaltates or graphite materials in the layer direction during charge and discharge. Accordingly, to inhibit the expansion and contraction is a crucial factor for achieving a longer cycle life in such battery systems.
In view of the foregoing, an object of the present invention is to provide a battery system that can theoretically almost completely eliminate the expansion and contraction in volume during charge and discharge. In this regard, not only the combination of positive and negative electrode active materials but also the combination of positive and negative electrodes is important.
As described above, in the fields of automobiles and backup power supplies where lead-acid batteries are currently used, the replacement of lead-acid batteries with lithium ion batteries will be necessary in the future.
Given the above, the present invention also provides a battery system that can provide an appropriate voltage for those applications. Thereby, a reduction in size and weight can be achieved. The term “appropriate voltage” used herein means a discharge voltage of 2 V, the same as that of conventional lead-acid batteries. As the power supply for idle stop of automobile, the battery is desired to be maintained in, for example, a 60 to 70% charged condition. This is suitable for charge control during regenerative charging. In order to make such control easier, it is important that the battery voltage should be changed relatively linearly in this range.
An object of the present invention is to provide a battery system which exhibits a flat discharge voltage during discharge, specifically, in the range where the battery capacity is not greater than 50% to provide a constant voltage of 2 V and also exhibits a linearly increasing voltage during charge in the range where the battery capacity is not less than 60% so as to make the charge control easier, which will be described later in EXAMPLE. There is no proposal from such viewpoint in the prior arts.
In the present invention, an extensive study was also made on electrolytes. The solvents unable to be used due to reductive decomposition in the case of using a conventional carbonaceous material include solvents having high oxidation resistance and those having a high flash point. The present invention discloses that these solvents are used yet the safety and reliability of the battery can be improved remarkably. Particularly, a great effect can be expected from the use of an ionic liquid. The inclusion thereof extremely reduces the possibilities of combustion and smoke in the battery because ionic liquids do not have a vapor pressure.
As for the separator, currently used microporous films made of olefin such as polyethylene and polypropylene can be used. These separators, however, are costly.
Since a battery is the combination of positive and negative electrodes, the balance of the capacity of positive and negative electrodes largely affects the performance of the battery, particularly, cycle life and long-term reliability. The present invention also examines the know-how regarding the capacity design and proposes a preferred range.
The current collector of the negative electrode is typically made of copper, but copper has a high specific gravity and poor weight efficiency. Besides, the use of copper is accompanied by the problems that it dissolves when the potential exceeds 3 V relative to that of lithium metal during deep discharge and that lithium ions are deposited in the form of a dendrite as a metal lithium during overcharge, which significantly impairs the safety. Despite the above problems, copper metal have been used because the negative electrode requires a carbonaceous material.
In the present invention, aluminum or an aluminum alloy can be used for the negative electrode core material to achieve weight reduction. It is also possible to prevent the battery safety from being significantly impaired during overcharge because aluminum absorbs lithium to prevent lithium from being deposited.